Gears are frequently used in automotive transmissions and the like. In recent years, there has been a demand for improvement in gear machining accuracy for the purpose of reducing the vibrations and noises of transmissions. Generally, gear machining involves performing gear cutting on a prescribed gear blank to form a tooth profile, performing heat treatment on the gear after the gear cutting, and then performing finishing (grinding) to remove strain and the like resulting from the heat treatment. Usually, each tooth surface of a machining-target gear is ground by bringing the machining-target gear after its heat treatment and a grinding wheel into mesh with each other at a crossed axes angle. As tools to be used in such grinding methods, there are tools of various shapes such as externally toothed shapes, internally toothed shapes, and threaded (worm) shapes for the shapes of gears to be ground.